This invention relates to a series-pass circuit for coupling a DC voltage supply subject to over-voltage transients to a high current electrical load.
In a motor vehicle electrical system, over-voltage protection circuitry is needed to protect electrical loads from damage due to over-voltages that can occur during jump-starting and load-dump conditions. Although passive shunt suppression devices such as Zener diodes or MOVs can be used in certain applications, the transient over-voltage energy can be too high to clamp with shunt devices, particularly in heavy duty vehicle applications. Although it is possible to use a series-pass suppression device such as a linear transistor instead of a shunt device, conflicting design requirements typically rule out the series-pass approach. Specifically, the series-pass suppression device must exhibit very low on-resistanice during normal operation, while exhibiting fast response to transient over-voltages to prevent over-voltage shoot-through to the electrical load. Accordingly, what is needed is a series-pass over-voltage suppression circuit that exhibits low on-resistance during normal pass-through operation, and that has the ability to quickly transition to a limited conduction mode in response to a detected over-voltage.
The present invention is directed to an improved series-pass over-voltage protection circuit including at least one N-channel enhancement mode MOSFET (NFET) coupling a DC voltage supply such as a motor vehicle storage battery to one or more high current electrical loads. The drain of the NFET is connected to the positive terminal of the DC voltage supply, and a high impedance gate voltage power supply biases the NFET to a fully enhanced state in normal operation to provide very low pass-through on-resistance. A gate discharge circuit including a high current capability transistor connected between the NFET gate and ground potential is activated in response to a detected over-voltage condition, and a compensation network having low AC impedance relative to that of the NFET is connected in parallel with the gate discharge circuit, providing a sink for the NFET gate charge to limit overvoltage shoot-through while the gate discharge transistor is activated in response to the detected over-voltage condition to quickly discharge the gate capacitance and transition the NFET to a limited conduction mode for regulating the load voltage.